FIG. 1 illustrates an example of a conventional current sensor applied to sensing rotation of a motor. The operation of this sensor is as follows.
The current sensor is constructed such that a high frequency signal generated by a signal generator 2 is transmitted from the primary winding N1 of a transformer T to the secondary winding N2 which is formed from a portion of a motor 1 current line, and is also transmitted through the tertiary winding N3 to a receiver R0. When a current flows in the motor drive circuit which comprises a power source E a switch SW and the motor 1, then the core C of the transformer T which is made from a saturable magnetic body, becomes saturated causing a drop in the level of the high frequency signal transmitted to the receiver R0 side. Hence the output of the receiver R0 which comprises an amplifier 3, an envelope detector 4 and a level tester 5, becomes an output condition of logic value 0 indicating that the motor is rotating. When a current does not flow in the motor drive circuit, then the core C no longer becomes saturated so that the high frequency signal is transmitted to the receiver R0 side without a drop in level. Consequently the output from the receiver R0 becomes an output condition of logic value 1 indicating that the motor is stopped. Moreover, a resistor R is provided in parallel with the motor 1, so that while the motor 1 is rotating under inertia immediately after switching off the switch SW, the current generated in the motor 1 flows through the transformer T via the resistor R and the transformer T becomes saturated. Thereby, also when the motor 1 is rotating under inertia, a signal of logic value 0 indicating that the motor 1 is rotating is produced from the receiver R0.
The abovementioned conventional current sensor however has the disadvantage in that, since if a disconnection fault occurs in the resistor R this cannot be detected, there is the possibility of erroneous generation of an output indicating that the motor is stopped, from the receiver R0 when the motor is still rotating under inertia.
Other methods for detecting whether or not a motor is rotating are the method wherein a drilled rotation disc for example is attached to the rotor of the motor, and rotation of the disc is detected using a light projector and receiver fitted on either side of the disc at the location of the aperture (the general method based on an encoder), or the method based on a tachometer generator.
The motor rotation detection methods based on an encoder or a tachometer generator however both extract a rotation output signal. Therefore, devices other than a device which is required to rotate the motor are required. For example with the encoder method a projector and receiver, and a disc for operating the projector and receiver are required, while with the tachometer generator method, a winding for power generation is required inside the motor.
It is a first object of the present invention to provide a current sensor which can, in the event of a fault occurring in a lead carrying a current being detected, reliably detect this fault. Moreover it is a second object of the present invention to provide a motor rotation sensor which can detect motor rotation without using any device which does not participate with the motor rotation.